Study deals with the impacts of heatwaves and droughts on European forests between 2018 and 2022It highlights that these events are becoming more frequent due to human-induced climate change. activities and affect both human well-being and ecosystem functioning. However, the intensity and effects of these events vary across the continent, making it crucial for decision-makers to understand the spatial variability of drought impacts. The study highlights that data on drought-related damages are currently scattered across scientific publications, government reports and the media.

The main objective of the study is consolidate data on damage caused by drought and heat waves in European forests between 2018 and 2022For this purpose, pan-European datasets on defoliation of tree crowns, insect damage, burned forest areas and loss of tree coverThe analysis was performed on data from 16 European countries, which were divided into four regions: northern, central, alpine and southernThe results were then compared with the reference period of the years 2010 to 2014.

The paper introduces two main approaches to studying extreme droughts. The first is paleoclimatic perspective based on proxy data, while Büntgen et al. (2021) found that recent extreme droughts (2015–2018) were unprecedented in the Czech Republic and neighboring regions over the past 2000 years. The second approach is long-term climate model projections, while Ionita et al. (2021b) suggest that megadroughts in the 15th, late 18th and early 19th centuries were longer and more severe in Europe. However, the authors point out methodological and regional differences between these studies, which make it difficult to draw firm conclusions.

In the section dedicated to damage to forests in the central zone in 2018–2022 The situation in is discussed in detail in Poland. Poland has a forest area of 9,242,000 ha (Central Statistical Office, 2017). In 2018, drought significantly weakened the condition of forests on an area of 43,500 ha and forest damage was observed on 29,400 ha (Jabłoński et al., 2019a, b). In 2019, the order of species from the healthiest to the most damaged was determined based on three parameters (average defoliation, proportion of healthy trees up to 10 % defoliation and proportion of damaged trees above 25 % defoliation): Fagus sylvatica, Alnus spp. < Fir < other deciduous, other coniferous Scots pine < Betula spp. < Spruce fir < Quercus spp. (Wawrzoniak, 2019). In 2020, signs of weakened or damaged forest stands caused by water regime disturbances, mainly due to drought, were reported in 253 out of 430 (i.e. 59 %) forest administrations (Lech, 2021).

The study also shows that pests that were considered minor in Polish forests a few years ago are now causing the death of many hectares (Sierota et al., 2019). The drought of 2015–2019 activated secondary factors leading to the death of pine stands (which represent 58.2 % of Polish forests) (Sierota et al., 2019). The following pests played a key role: the pine bark beetle (Ips acuminatus), mistletoe (Viscum spec.), pine rust (Sphaeropsis sapinea), pine beetle (Phaenops cyanea), common rhizome (Heterobasidion annosum) and the support (Armillaria spp.) (Sierota and Grodzki, 2020). Observations in Poland indicate a significant correlation between drought and the abundance of the pine bark beetle (Ips acuminatus) (Jabłoński et al., 2019a, b; Plewa and Mokrzycki, 2017), a species that until recently was not considered a significant forest pest (Głowacka, 2013). The occurrence of mistletoe (Viscum sp.) was also underestimated. After long periods of drought, the area of coniferous (mainly pine) forests heavily infested with mistletoe drastically increased from 1400 ha in 2017 to almost 23,000 ha in 2018 (Jabłoński et al., 2019a). Mistletoe was found on 14 forest tree species, with fir and pine being the most infested and birch, as well as mixed deciduous species and spruce (Lech et al., 2019). In addition, well-known forest pests, such as the spruce bark beetle (Ips typographus), continue to pose a major threat to Polish forests. The decline of spruce stands in Central and Eastern Europe was already increasing in the 1970s and 1980s (Sierota et al., 2019). After the drought in 2015, spruce stands continue to decline, with new outbreaks of the wood borer affecting stands in the Western Carpathians and the Sudeten Mountains. The ongoing climatic conditions combined with high populations of the wood borer pose an extremely high risk of further outbreaks (Grodzki, 2010).

Recent years have seen significant losses in state forest land due to drought and high temperatures (DGLP; Dyrekcja Generalna Lasów Państwowych). Drought-related areas amounted to 40,852 ha (2018), 60,356 ha (2019), 58,056 ha (2020), 34,673 ha (2021) and 20,258 ha (2022). Losses caused by high temperatures (burns, wilting, dying) were reported at 80 ha (2018), 340 ha (2019), 2,574 ha (2020), 197 ha (2021) and 244 ha (2022). The prolonged drought in Poland has also led to a decrease in surface and groundwater levels, as well as a decrease in tree growth, stand vitality and resistance to pathogens and pests (Kwiatkowski et al., 2020). Species affected by this process include oaks, for which the impact of the falling groundwater level has been observed since the late 1980s (Przybył, 1989). Current fluctuations in groundwater levels further weaken oaks and accelerate their decline (Skrzecz et al., 2022), e.g. on the Krotoszyń Plateau (Danielewicz, 2016).

IN Czech Republic forest disturbances, mainly caused by pests, were triggered by drought and high temperatures. Near Kostelec and Černé Lesy, studies found that the abundance of the bark beetle was related to the duration of April sunlight in the previous year and the average annual air temperature in the current year (Pirtskhalava-Karpova et al., 2024). In the Šumava National Park, researchers observed that the surface temperature in attacked stands was higher in the year preceding pest colonization compared to intact stands (Kozhoridze et al., 2023). At the beginning of the extensive bark beetle attacks, spruce accounted for 50.5 % of stands and pine for 16.4 % (Zahradník and Zahradníková, 2019). This abundance of trees susceptible to bark beetles has led to the assumption that the Czech Republic may have been the epicentre of the European bark beetle outbreak (Hlásny et al., 2021), as more than 50 % of Czech forests were seriously threatened by this pest, leading to high ecological and economic losses (Fernandez-Carrillo et al., 2020). The volume of wood harvested annually is approximately 15 × 10⁶ m³, of which approximately 1 × 10⁶ m³ is wood infested with insects (WII). In 2018, 25.6 × 10⁶ m³ was harvested and 13 × 10⁶ m³ was WII; in 2019, this was 32.5 × 10⁶ m³. On the German side of the border, an area of approximately 150 ha was affected in the Saxon Switzerland National Park (DAV, 2022). During the decade 2010–2020, almost 100 × 10⁶ m³ of solid wood was harvested in the Czech Republic in connection with bark beetle attacks, resulting in financial losses to Czech forestry of approximately EUR 1.12 billion (Toth et al., 2020). More than half of this volume has been harvested since 2017, and this amount of unplanned disaster logging represents an approximately 3-fold increase from 2017 to 2018 (Moravec et al., 2021). During the dry season (2015–2019), there were also clear signs of loss of vitality, with a reduction in the growth of five main species due to drought compared to the reference period 2005–2009 (Jiang et al., 2024).

In the section about forest damage in the southern zone in 2018–2022 It is stated that although Portugal has recently shown an increasing frequency of droughts associated with heat waves (Bezak and Mikoš, 2020; Vogel et al., 2021; Ribeiro et al., 2020), which leads to a deterioration of the limiting climatic conditions for plant growth, the situation of maritime pine (Pinus pinaster, one of the most common species) is not entirely unfavourable according to Kurz-Besson et al. (2016). Detailed information on defoliation and insect-damaged wood in Portugal has not been available since 2006 (ICP Forests, 2007).

In the chapter on Persistent drought in groundwater It is emphasized that to assess the link between drought persistence and groundwater persistence from observations, it is necessary to know the state of groundwater (including soil water) in space and time. Groundwater status is usually observed in boreholes through in situ measurements of the groundwater table or piezometric head. These measurements provide information at a point scale in space and usually with a low temporal frequency, as they are usually carried out manually and are therefore not continuously recorded. This leads to discontinuous pictures of groundwater status in space and time, which are commonly interpolated using models, inversion and data assimilation. However, it should be noted that there is no comprehensive database of groundwater observations for the whole of Europe or for specific countries. The data remains fragmented and scattered across many political and private institutions and is not publicly available. This makes it impossible to formally analyze this nexus within the scope of this study and only general principles can be discussed.

Finally, the document provides information about special editionin which this article is published, as well as Acknowledgements and information about financial support research. Spring